Power brake assist using differential pressure actuated brake boosters is standard in the motor vehicle industry. These brake boosters typically have a diaphragm separating a vacuum chamber always open to a source of vacuum, such as created in an engine intake passage, and a working chamber normally provided with vacuum but controlled by valve apparatus responsive to input brake pedal actuation to allow atmospheric air therein and thus provide brake force assist through a differential pressure across the diaphragm. In some systems, it is also known to provide a vacuum pump, either in place of the engine vacuum source or as a backup therefor, to provide a normal supplied vacuum.
The braking assist force provided by such known vacuum brake boosters is only an assist force, which is added to the force provided by the vehicle operator through the brake pedal. The vehicle operator applied force is itself transmitted directly through the booster apparatus and applied to the master cylinder, whether or not the assist braking force is generated by the vacuum brake booster. Thus, vehicles provided with such vacuum brake boosters may be stopped even in the unlikely occurrence of loss of vacuum, producing reduced or non-existent braking assist force, although reduced braking force can result in longer stopping distance. Braking systems are designed for safe stopping within specified distances at predetermined vehicle speeds, even with no braking assist; but this task is difficult for heavier vehicles, and a source of back-up vacuum is desired in some cases to ensure that such vacuum based braking assist force is not lost. The object of this invention is to provide a self-contained, compact, light-weight, backup vacuum assist apparatus, and particularly such apparatus that is capable of mounting directly to a vacuum brake booster without necessity of additional vacuum hoses and with minimal additional external electrical wiring requirements. Such apparatus is capable of helping large, heavy vehicles to meet the standards of FMVSS 135.